In-vehicle accelerometer devices refer to devices that are installed somewhere on or in a vehicle and contain accelerometers. Personal navigation (PN) devices and automatic crash notification (ACN) devices are examples of in-vehicle devices that may contain a multiple axis accelerometer sensor. These in-vehicle accelerometer devices may either come with the vehicle from the factory or be installed aftermarket, e.g., by the vehicle owner or a technician. Although factory installed devices typically have known accelerometer/device mounting angles relative to standard vehicle body axes, the aftermarket in-vehicle accelerometer devices may end up with mounting angles that are unknown to the manufacturer of said device. These aftermarket in-vehicle accelerometer devices may consequently need a system and method for estimating the accelerometer/device-to-vehicle mounting angles so that the accelerometer sensor data can be transformed into the standard vehicle body coordinates to allow a standard analysis of the orientation sensitive data.
A very accurate accelerometer sensor-to-vehicle-body coordinate transformation may be required, for example, for aftermarket PN devices to function at an intended high level of performance. Some personal navigation devices use multiple axis accelerometers and gyroscopes to enable an inertial navigation assist to GPS navigation. The combined inertial and GPS navigation mode provides better location information when going through areas of poor GPS signal reception. Accurate coordinate transformation is required since navigation with accelerator-only inertial clusters has the property that errors in acceleration are twice integrated into errors in position—a process that exponentially amplifies the errors. Mounting angle estimation techniques appropriate for aftermarket PN devices is discussed by Eric Vinande, Penina Axelrad, and Dennis Akos in the article, “Mounting-Angle Estimation for Personal Navigation Devices”, IEEE Transactions on Vehicular Technology, Vol. 59, No. 3, March 2010. The techniques presented use GPS for determining forward or rearward vehicle acceleration and vehicle orientation and are capable of estimating the pitch, roll and yaw mounting angles to accuracies within 2 degrees. This accuracy allows a ‘reasonable’ 10 second inertial navigation to have a 9 meter error. We again note that the nature of this type of inertial navigation for GPS coverage gaps is that the errors will grow exponentially without a timely ‘truth’ reference such as an independent GPS location estimate.
Other in-vehicle accelerometer devices, may function fine with less accurate accelerometer sensor-to-vehicle-body coordinate transformations. Aftermarket ACN devices, for example, may fundamentally only be required to identify the ‘crash impact angle’ to within an angular quadrant, i.e., to identify the direction of the collision impact as being from the ‘front’, ‘passenger's side’, ‘rear’ or ‘driver's side’ of the vehicle. This limited resolution crash impact angle estimation requirement for some ACN devices allows the mounting angle estimation errors for those ACN devices to be greater than the 2 degrees or so that are associated with the PN devices. Generally, in-vehicle accelerometer devices that do not use the accelerometer data for inertial navigation will have less stringent requirements for mounting angle estimation errors than those that do.
A problem with the mounting angle estimation techniques described by Vinande et al. is that since they are focused on aftermarket PN devices, they assume the availability of a functioning GPS location subsystem in the device and impose processing requirements on the use of the GPS location data for the purpose of angle estimation. The GPS data is used to estimate accelerations, horizontal road conditions and to determine appropriate times for the pitch and roll phase and the subsequent yaw phase of the angle estimation methods. Mounting angle estimation procedures that do not require a GPS unit are also desired for in-vehicle accelerometer devices.
Another problem with known mounting angle estimation methods is that the accelerometer/device and vehicle body reference axes are assumed to be favorably oriented for the equations used to estimate the angles. An automatic means of detecting unfavorable device accelerometer axes assignments and performing axes reassignment is desired to improve the accuracy of the mounting angle estimations.
What would be optimal is for the mounting angle estimation and compensation methods for in-vehicle accelerometer devices to be conducted without being dependent on the presence of a GPS subsystem and are capable of automatic reassignment of the accelerometer axes for devices with unfavorable orientations.